Nowadays, a mass of RF/Microwave modules are designed for portable terminals such as handsets, e-readers and tablet PCs. This trend motivates the research of high integration techniques for saving board space, decreasing system costs and simplifying the design effort, especially in the designs of microwave passive components because they occupy most of circuit area. In the past few years, much effort has been paid to offer several effective solutions for high integration techniques. Among them, the combination of two or more independent function circuits into one circuit is one of the popular approaches. For example, the integration of balun and bandpass filter (BPF) not only exhibits the unbalanced-to-balanced conversion, but also bandpass filtering.
In order to bring in fine bandpass response, a variety of resonators are researched. For example, many balun BPFs are evolved from the classic quarter- and half-wavelength resonators with folding topology or the single dual-mode resonators are employed to construct the compact balun BPFs. To cater to the dual-band wireless systems, plenty of research focuses on the balun BPFs with two passbands. To extend the ability of the microwave components for supporting multiple frequency bands, tunable or reconfigurable techniques have drawn much attention for research and development because of their increasing importance in improving the capabilities of current and future wireless communication systems. Accordingly, many tunable BPFs have been under intensive development, but relatively little research has been done on the tunable balun. In particular, up to now, the study concerning the frequency tunable filtering balun with bandpass response is rather sparse.